1. Field of the Invention
The present invention relates to a variable-speed permanent magnet rotating electric machine.
2. Description of the Background Art
A conventional variable-speed permanent magnet motor is disclosed in Japanese Patent Application Publication No. 2003-9486, for example. This motor is of a double-armature type provided with a generally ring-shaped rotor and a pair of armatures, in which one of the armatures is located inside the rotor and the other outside the rotor so that electrical angular positions of the inner and outer armatures can be relatively varied.
Specifically, in this motor of the Publication, the angular positions of the inner and outer armatures disposed concentrically inside and outside the rotor, respectively, are made relatively variable. The motor is controlled by a so-called field-weakening control method in which magnetic fields linked on each armature do not add up to a maximum level of magnetic flux but cancel out each other in part to decrease a total magnetic flux level. Field-weakening control serves to decrease combined voltages induced in armature coils of the two armatures in a high-speed range so that the voltages induced in the armature coils would not exceed a maximum voltage applicable to the armature coils. This arrangement makes it possible to easily perform variable-speed control operation of the motor and manufacture the motor with reduced electrical limitations and an increased degree of freedom of design.
As stated above, a conventional double-armature rotating electric machine has two stator-side armatures one of which is located inside a rotor and the other outside the rotor. Therefore, this type of rotating electric machine typically employs a structure in which the rotor is supported in a cantilever fashion. For this reason, it is difficult to hold the rotor in a stable position when the rotor is rotating. More specifically, although the rotor can be easily sustained by a cantilever support structure if the rotor is of a type having a small axial length, it is difficult to hold the rotor in a stable fashion if the rotor has a large axial length. A previously known arrangement for supporting the rotating rotor in a stable fashion is to support the rotor by a pair of bearings or the like located at both ends of a rotor shaft. This prior art approach has a problem that a considerably complicated rotor support structure is required.